The Cassini RADAR instrument, operating in its scatterometry
mode, obtained continuous-wave (cw) echo power spectra from
Phoebe during the inbound and outbound legs of the flyby, 4
h before and 2.5 h after closest approach. Phoebe's distance
and subradar coordinates were approximately (93,000 km, 247
deg W, 26 deg S) inbound and (56,000 km, 323 deg W, 26 deg
N) outbound. The durations of the cw sequences were 6 and 5
minutes. Larger intervals in the RADAR windows were devoted
to observations with a chirp waveform able to provide range
as well as Doppler resolution, and to passive radiometry;
those data are not yet reduced.

For Phoebe (and Dione, Mimas, Iapetus, Enceladus, Rhea,
Hyperion, and Tethys), scatterometry aims to use estimates
of radar albedo and angular scattering law to constrain the
near-surface bulk density and/or the relative cleanliness of
the icy regolith. The RADAR instrument's wavelength is 2.2
cm, vs. 3.5 cm or 13 cm for most groundbased radar
astronomy, but Arecibo and Goldstone observations of the icy
Galilean satellites and of asteroids give us no reason to
expect significant wavelength dependence in this regime.
Comparison of RADAR measurements of Titan and Iapetus with
groundbased results will let us evaluate this expectation
and will be key to calibrating both our measurements and
their interpretation.

Our inbound and outbound Phoebe echoes indicate Lambertian
scattering, which requires structural complexity at scales
no smaller than a centimeter. However, despite Phoebe's
prominent large-scale topography, our spectra are nearly
featureless, suggesting that the radar roughness is
sub-topographic. We probably are seeing a combination of
single scattering and multiple scattering from surface and
subsurface structure. At this writing, our calibration
indicates that Phoebe's average radar albedo is much closer
to that of Iapetus than to those of the icy Galilean
satellites.